A cyclone separator is a common apparatus used to concentrate particles suspended in a carrier fluid. This apparatus, commonly referred to simply as a cyclone, is a frustum-shaped and cylinder-shaped apparatus with a single vertical axis, wherein the large diameter end of the frustoconical portion is typically oriented directly above the small diameter end, and the cylindrical portion is above the frustum, such that the walls of the cylinder and frustum are contiguous. It is worth noting that although the term "cyclone" is applicable to all three types of suspension systems, "hydrocyclone" is generally limited to liquid-particle and liquid-liquid systems.
For currently known cyclones, a pressurized feed provides the necessary potential energy that is converted to rotational motion by introduction through a tangential inlet near the top of the cyclone's upper cylindrical section. The suspension spirals along the outer wall creating centrifugal acceleration on the entrained particles, forcing them outward. As the suspension travels downward it encounters the lower conical section where the suspension accelerates as the cross-sectional area decreases. The degree of separation is based on particle specific gravity, particle size, particle shape, fluid specific gravity, and viscosity.
The centrifugal acceleration in a cyclone plays a decisive role in its ability to classify particles in suspension. A settling particle in a cyclone has three forces in equilibrium acting on it: a centrifugal force from rotational motion; a buoyant force from differences in particle and fluid density; and a drag force from fluid friction. These three forces are functions of velocity, with buoyancy and friction opposing the centrifugal force.
Larger particles will experience a greater centrifugal force and will gravitate towards the outer wall while smaller ones will be drawn into the inner vortex. The majority of these smaller particles are pumped out of the cyclone through an overflow outlet placed in the upper central portion of the cyclone. Some particles in the inner vortex are caught in eddy flows and become remixed with the carrier fluid.
Residence time in the cyclone has to be considered in determining performance. Factors that affect performance can be grouped into two categories. One is operating variables consisting of flow rate and feed composition. Higher flow rates, though lowering residence time, produce higher yields due to increased shear with increased pressure drop but at the expense of an exponential increase in energy consumptions. The other is design variables associated with geometry that affect efficiency by determining flow patterns and are directly related to frictional and turbulent losses.
Cyclones have several advantages over other separation apparatus. First, they can accommodate and separate large volumes of suspension in a relatively short period of time. Higher capacity or finer fractionation of particulate can be achieved by linking multiple cyclones in parallel or series, respectively. Cyclones can also be used in combination with other separators to enhance thickening or to increase overall mass recovery. Second, cyclones generally do not have filters which are subject to clogging. Third, a cyclone is a simple device that lacks mechanical moving parts; thus, it is relatively easy and inexpensive to manufacture and maintain. On the other hand, one of the main disadvantages of the cyclone is that the separation is not as sharp as compared to filters, for example.
Crossflow filters are also known. These filters provide particulate-free filtrate and the ability to combat, or at least slow down, dead-end filtration, cake growth, which is predominant in fine particulate solution separations, are the major advantages to using crossflow filtration. Ultrafiltration, reverse osmosis, microfiltration, and thickening of solid/liquid solutions using anisotropic membranes, microporous media, and tightly woven material, respectively, are several of the areas that use crossflow filtration.
Typically, crossflow filters consist of two long concentric tubes. The inner tube has a porous stationary media surface and the outer tube is non-porous. A suspension is introduced into the inner tube under pressure. Some of the carrier fluid is then forced out of the inner porous tube into the outer tube, thereby concentrating the suspension within the inner tube. Shear force is developed by flow parallel to a stationary media surface which removes most particulate buildup. Over time, however, a residual cake forms, inhibiting filtrate flux and requiring either a cyclic back flush or another means of media cleaning.
Some manufacturers and research facilities, in an effort to minimize the filtrate flux decline due to cake buildup, have devised high-shear crossflow filters. These filters are necessary for applications where filtrate flux is too low, backwashing frequency is excessive, and concentration of particulate is too high for low-shear crossflow filtration to be effective.
Although the separation force is the same for both high-shear and low-shear crossflow filtration, the method of generating the necessary shear is quite different. In low-shear the fluid pressure provides the necessary velocity as the solution is pumped through microporous tubes or between plates utilizing various porous media. High-shear crossflow filtration relies on mechanical rotational energy from either rotating disks or axial filters to impart a large velocity gradient to the fluid that provides the shearing forces eliminating the need for a pump induced pressurized feed. Prototype high-shear filters are designed to allow operation of several separation stages in either parallel or series operation depending on the application. A rotating disc or rotating axial cylinder are the two main geometries of introducing a high shear to a fluid with a filter medium applied to any surface.
Filter flux through the media decreases over time because media blockage occurs by particles smaller than pore size, and a secondary membrane develops which slowly increases in depth and density. Decrease in flux reduces yield and increases downtime due to cleaning or changing filter media. To maximize operation and yield, intricate backwash systems can be designed for the system to remove excessive dynamic membranes that normal aid in separation. This further complicates the design and increases the cost of the already expensive high-shear filter.
Therefore, there is a need for a fixed filtering apparatus that is capable of a high degree of separation, can quickly separate large quantities of suspension, and is inexpensive to manufacture and maintain.